Vapor generator with gas recirculation



April 1966 E. M. POWELL ETAL 3,246,635

VAPOR GENERATOR WITH GAS RECIRCULATION Filed April '7, 1965 3Sheets-Sheet 1 FIG- April 1966 E. M. POWELL ETAL 3,246,635

VAPOR GENERATOR WITH GAS RECIRGULATION Filed April 7, 1965 5Sheets-Sheet 2 FIG-4 4% A ril 19, 1966 E. M POWELL ETAL 3,246,535

VAPOR GENERATOR WITH GAS RECIRCULATION Filed April 7, 1965 3Sheets-Sheet 5 FIG 5 United States Patent 3 246 635 VAPOR GENERATORWITII GAS RECIRCULATION Elno M. Powell and Robert H. Wolin, Avon, Conn.,as-

signors to Combustion Engineering, Inc., Windsor,

Conn., a corporation of Delaware Filed Apr. 7, 1965, Ser. No. 446,377 11Claims. (Cl. 122479) This application is a continuation-in-part of ourprior co-pending application Serial No. 331,751 filed on December19,1963, now abandoned.

This invention relates to steam generators which burn high moisturecontent 'coal and in particular to a method and apparatus for using gasrecirculation for steam temperature control therein.

High moisture content coals such as brown coal require substantialdrying before they can be introduced into a furnace. This isaccomplished in the mill which simultaneously pulverizes and dries thecoal, an integral part of this mill being apaddle wheel fan orblower ofsome sort. In order to obtain sufficient heat for adequate drying, gasfrom the furnace is admitted to the mill suction in quantities requiredfor satisfactory drying. The remainder of the gas which the mill drawsis supplied by preheated air taken from the air stream after the airheater. A characteristic of such mills is that the volume air flowleaving the mill is essentially constant regardless of the quantity offuel being pulverized.

Brown coal not only has a high moisture content but usually a high ashcontent as well. This in combination with the large quantity of inertgas which is being supplied through the mill creates a situation whereit is very diflicult to maintain proper ignition of the fires.Introduction of recirculated gas into the furnace for steam temperaturecontrol would further increase the quantity of inert gas and aggravatethis already difficult situation of flame stability. Superheat andreheat temperature control on such jobs has therefore been limited toalternative means such as gas bypass and fluid-to-fluid heat exchangers.

In the instant invention gas recirculation is made possible byselectively supplying gas to the mill from various sources therebyconcurrently obtaining the benefits of high gas temperature for dryingand steam temperature control via gas recirculation.

It is an object of the invention to provide an improved vapor generator.

Another object of the invention is to provide a practical means of steamtemperature control using gas recirculation in steam generators burninghigh moisture coal.

A further object is to maintain satisfactory drying and therefore propermill air temperatures while firing high moisture coal.

Other and further objects of the invention will become apparent to thoseskilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the resultsdesired, as hereinafter more particularly set forth in the followingdetailed description of illustrative embodiments, these embodimentsbeing shown by the accompanying drawings wherein:

FIGURE 1 shows a preferred embodiment of the invention where a gasrecirculation fan is used to move the flue gas from the rear pass of theboiler to the mill;

FIGURE 2 illustrates the source of the gas flow which ice is leaving themill when there is no recirculation through the duct from the convectionpass;

FIGURE 3 shows the source of gas which is leaving the mill when flue gasis recirculated from the convection pass for steam temperature controland hot air is introduced into the mill;

FIGURE 4 shows the source of gas which is leaving the mill when flue gasis recirculated from the convection pass for steam temperature controland cold air is introduced into the mill; and

FIGURE 5 illustrates another embodiment of the invention which differsfrom FIGURE 1 in that all motive power for recirculation is supplied bythe mill, thereby omitting the gas recirculation fan, and dampers areinstalled in the furnace gas recirculating duct.

FIGURE 1 illustrates a steam generator having a furnace 2 surrounded byradiant heating surface 3. Fuel enters through burners 4 and air throughwindbox 5 so that combustion takes place in the furnace 2. Hotcombustion ga'ses then pass through horizontal flue 7 thence throughvertical flue or rear gas pass 8 passing out of the unit through airheater 9 as cooler gas.

Disposed within the steam generator is low temperature reheater 10 andhigh temperature reheater 12 with steam passing therethrough to bereheated, returning to a steam turbine (not shown) from header 13. Therear gas pass 8 also includes economizer surface 14. Superheating isaccomplished in low temperature superheater 15 with steam being conveyedthrough pipe 17 to high temperature superheater 18 from which it isconveyed to the steam turbine (not shown) through steam pipe 19.

Combustion supporting air is supplied from forced draft fan 20 throughduct 22 and through air heater 9. This air may then be supplied throughduct 23 to windbox 5 from which it is delivered to the furnace 2 ascombustion supporting air. Cold air ahead of the air heater may bewithdrawn through duct 24 for supply to the mill 25, while hot air maybe withdrawn from duct 23 through duct 27 to mill 25. The pressure ofthe air at each of these locations .is higher than the furnace and millsuction pressures.

Furnace gases may be recirculated from the furnace 2 through the furnacegas recirculating duct'28 to mill 25, while cooler gas from the rear gaspass 8 may be recirculated through duct 29 and gas recirculating fan 30reaching the mill through duct 32. Fuel being supplied to the mill froma feeder (not shown) together with the drying gases is conducted fromthe mill through coal pipes 33 to burners 4. The temperature of thismixture, known as mill air temperature, is maintained by controlling therelative quantity of these gases entering the mill.

Operation of the unit without the use of gas recirculating duct 29, orair duct 24, is as follows. Mill supplies through pipe 33 the amount offuel which is being fed to the mill plus an essentially constant volumeof gas. At high loads where considerable drying is required the milltemperature tends to be low, whereby air damper 34 is closed downdecreasing the relatively cool air supply to the mill. The mill takingessentially constant flow draws more hot furnace gas through duct 28,thereby raising the mill temperature. Thus the preheated air suppliedthrough duct 27 is regulated such that the desired temperature isobtained in the gases leaving the mill. As load is decreased therequired drying decreases, and the mill temperature tends to increase.The air damper 34 is then opened admitting more relatively cool air andthe flow of recirculated furnace gas is decreased.

This is illustrated in FIGURE 2 where the abscissa is coal supplied tothe mill as percent of rating and the ordinant is the volume of gasesleaving the mill. The coal quantity is, in general, in proportion tosteam generator output. Line 40 illustrates the total volume leaving themill which is essentially constant for any capacity. The space betweenline 40 and line 41 shows the amount of vapor in the gases leaving themill as a result of the coal drying, where the space between lines 41and lines 42 shows the portion of the gas leaving the mill which issupplied'through the furnace recirculation duct 28. The remainderbetween line 42 and the base line shows the quantity of air which issupplied through duct 27.

The foregoing discussion concerned mills where the fan characteristicswere such that essentially constant volume flow occurs when the mill isfloating on an open recirculating system. If, however, other fancharacteristics are encountered, dampers may be installed in the furnacegas recirculating duct 28 to control this gas such that the flow leavingthe mill is constant.

It is well known that increased gas flow through convection sectionssuch as horizontal flue 7 and particularly in vertical flue 8 by meansof gas recirculation is effective to increase heat transfer in thoseareas thereby increasing the steam temperature leaving sections such asreheaters 10 and 12 and superheater 15. In the illustrated embodimentsgas recirculation is employed to control the steam temperature leavingreheater 12 through header 13, although it could equally be used tocontrol superheater temperature.

In this invention the flue gas which is recirculated for steamtemperature control through duct 29 is introduced into the mill 25. Thetemperature of this flue gas is lower than the furnace gas which isbeing supplied to the mill and higher than the preheated air which isbeing supplied to the mill. During this operation the mill stilldelivers the same volume of gas that it had previously, but since theflue gas is being introduced, the furnace gas and hot air must decrease.The hot air dampers 34 are controlled in the same manner as they werepreviously to hold the desired mill air temperature, while therecirculated furnace gas through duct 28 readjusts itself, oralternately may be controlled by dampers to maintain the proper flow.

The operation of the preceding paragraph may be obtained with thecontrol equipment illustrated in FIG- URE 1 wherein temperaturetransmitter 60 detects the mill air temperature in line, 33. Thistransmitter sends a control signal through control line 62 to controller63. This controller operates the damper 34 to maintain the preselectedtemperature in line 33. When this damper reaches its wide open position,acontrol signal is emitted through control line 64 to controller 65which operates to open control damper 26 so'that the temperature in line33 may be maintained. When damper 26 reaches a wide open position, acontrol signal is emitted from controller 65 through control line 66back to controller 63 and opreates to close damper 34 whereby anincreased percentage of the air is passed through the cold air duct 24.The steam temperature leaving reheater 12 is sensed by temperaturetransmitter 67 which emits a control signal through line 68 tocontroller 69. This operates to vary the speed of the gas recirculationfan 30 carrying the gas recirculation through duct 29 and accordinglyregulating the temperature of the steam leaving the reheater 12. Thepreviously described mill air temperature control system operates tomaintain the mill air temperature at a preselected value.

This is illustrated in FIGURE 3 where the desired flue gas quantity toobtain proper reheat temperatures is shown by line 43. The total volumeleaving the mill is again shown by line 40 and the vapor in the gasesleaving the mill is shown by the difference between lines 40 and 4 41.The hot air is reduced to that now indicated by the difference betweenlines 43 and 44, while the furnace gas is reduced to that indicated bythe difference between lines 41 and 44. At low loadings on the mill therecirculated flue gas may have to be reduced to avoid overheating of themill, as indicated by line 45.

The amount of recirculated gas required for steam temperature controldepends on the particular design of the steam generator. It is possiblethat in some designs more flue gas would have to be recirculated than inthe instant embodiment and therefore this problem of overheating thevmill may occur at higher loadings. A similar situation occurs wherefewer mills are used for low load operation, thereby increasing the gasrecirculation per mill. This overheating can be limited by introducingto the mill, in place of preheated air, the air ahead of the preheaterthrough duct 24. This is illustrated in FIGURE 4 where line 43 againshows the required flue gas for steam temperature control. Thediflerencebetween line 43 and line 47 shows the cold air requirements for the millwhile the difference between line 41 and line 47 shows the furnace gasrequirements. It can be seen that in this type operation less air isrequired and more furnace gas is required than is the case when hot airis being recirculated. It follows that more recirculated flue gas can betolerated before the furnace gas is decreased to zero.

Operation of this system in the preferred embodiment would involverecirculating gas to obtain proper steam temperature. The hot air to themill and the recirculated furnace gas would be controlled so that aconstant gas flow is held leaving the mill at the proper temperature. Ifthe mill temperature rises above the desired level and at the same timethere is no furnace gas being recirculated, cold air from ahead of theair heater would be introduced in place of the hot air to hold the milltemperature down to acceptable limits. It is desirable whenever possibleto use the preheated air for the mill since the additional air flowpassing through the air heater has a beneficial effect on steamgenerator efficiency.

Since all of these variables are intimately related, control of any ofthe controlled items could be initiated by any one of the controllingvariables; for instance, when it is desired to decrease the reheattemperature, the hot air dampers 34 could be opened thereby increasingthe flow through duct 27 and concurrently decreasing the flow throughduct 28, thereby decreasing the mill temperature, while the volumeleaving the mill remains constant. Recirculated gas flow would then bedecreased which would permit an increase in the recirculated furnace gasflowing through duct 28, which will return the mill temperature to thedesired level. In this way steam temperature is controlled bymovementof. the hot air dampers 34 while mill temperature is maintainedthrough control of therecirculating flue gas flow.

FIGURE 5 illustrates another embodiment of our invention where thesolemotive power for the gas recirculation is the mill itself. The generalstructure of the steam generator is similar to that of FIGURE 1, theonly difference being the arrangement of the gas and air ducts to themill. Control dampers 50 are included in duct 28 for control of therecirculated furnace gases. The flue gas is recirculated through duct 29being controlled by dampers 51. The hot air again passes through duct 27through dampers 34 to the mill while the cold air again passes throughduct 24 and dampers 26 to the mill.

When excess mill temperatures are obtained with recirculated furnace gasat zero, the cold air through duct 24 is again used. Considering, asillustrative the operation of only ducts 28 and 29 and 27, the damperson these three ducts are controlled to maintain total mill flow, milltemperature, and proper recirculation for reheat steam temperature.Dampers 51 may be opened to increase flue gas recirculation therebyincreasing reheat steam temperature while damper 34 controlling the hotair is decreased to restore the mill flow to its proper amount, andfurnace recirculating gas damper 50 may be closed to maintain the propermill air temperature.

The apparatus shown in FIGURE 5 may be controlled in a manner similar tothat of FIGURE 1 wherein the temperature transmitter 60 detects thetemperature of the mill air in pipe 33 and operates dampers 34 and 26 tomaintain the temperature at a preselected value. The reheated steamtemperature is detected by temperature transmitter 67 which emits acontrol signal through control line 68 to controller 70 which operatesdamper 51 to control gas recirculation in response to reheated steamtemperature. In this manner the reheated steam temperature may bemaintained at a preselected value. Flow meter 71 is connected acrossorifice 72 in line 33 to determine the total flow of mill air. A signalis emitted from this transmitter through control line 73 to controller74 which operates damper 50 to maintain the flow in line 33 at apreselected value.

The volume of gases leaving the mill may be varied with load as desiredin line with another mode of mill operation.

While we have illustrated and described a preferred embodiment of ourinvention it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and scope of the invention. Wetherefore do not wish to be limited to the precise details set forth butdesire to avail ourselves of such changes as fall within the purview ofour invention.

What we claim is:

1. A vapor generator comprising in combination a furnace having heatexchange tubes on its wall, a gas pass extending from the furnace, meansfor heating vapor having at least a portion thereof disposed in said gaspass, means for firing said vapor generator with a particulate fuel ofhigh moisture content and for regulating the temperature of the vaporheated in said vapor heating means comprising; a pulverizing means, theeffluent from which is conveyed to the furnace, means supplying saidfuel to the pulverizing means; means passing a hot gaseous mediumthrough the pulverizing means for fuel entrainment and drying includingmeans conveying thereto hot gases from the furnace, separate meansconveying thereto combustion gases which have traversed said vaporheating means, means conveying thereto air; and means regulating thesupply of gaseous medium to the pulverizing means in response to thetemperature of the vapor leaving said vapor heating means and to meetthe temperature and flow requirements of the mill.

2. A vapor generator including a fluid cooled furnace, vapor heatingmeans disposed so that gases produced in said furnace pass thereover,means for firing the furnace with a high moisture content fuel includinga gas swept pulverizing means the eflluent of which is conveyed to andintroduced into the furnace; means for supplying said pulverizing meanswith a high moisture content fuel and with carrying and drying gases,with these latter including air, hot gases from the furnace at alocation upstream of said vapor heating means, and gases which havetraversed at least a portion of said vapor heating mean-s; means forregulating the supply of gases which have traversed said vapor heatingmeans in response to the temperature of the heated vapor independent ofthe firing rate; and means for regulating the supply of other gases tothe pulverizing means in response to the temperature of the pulverizingmeans effluent.

3. An apparatus as in claim 2 where the means for supplying saidpulverizer means with carrying and drying gases including gases whichhave traversed at least a portion of said vapor heating means includes agas recirculation fan receiving gases which have traversed at least aportion of said vapor heating means and delivering these gases to saidpulverizing means.

4. An apparatus as in claim 2 Where the heating means comprises steamreheating surface.

5. In combination a steam generator for burning high moisture contentfuel and a mill for pulverizing and drying the fuel, comprising: afurnace for the combustion of the fuel, hot gases thereby being formed;an air heater; a flue conveying the hot gases to the air heater; steamheating surface disposed within the flue, for heating steam, and coolingthe gases; means for supplying air at elevated pressure to the airheater; means for conveying the heated air to the furnace; means forconveying hot gases from the furnace to the mill; means for conveyingcooled gases to the mill from the flue at a location downstream of atleast a portion of the steam heating surface, but upstream of the airheater, and means for regulating the flow of these cooled gases inresponse to the temperature of steam leaving said steam heating surface;means for conveying a portion of the heated air to the mill; means forconveying air at elevated pressure to the mill from a location ahead ofthe air heater; means for supplying fuel to the mill; means forconveying the mill products to the furnace; and means for regulating theflow of air to the mill in response to the temperature of said millproducts.

6. In a steam generator for burning high moisture content solid fuel,having a mill for pulverizing this fuel, and steam heating surfacedisposed within the steam generator, the method of operation comprising:burning fuel in a combustion zone, thereby forming hot gases; coolingthese gases by transferring heat to steam; determining a measure of thequantity of heat transferred to the steam; conveying a portion of thehot gases from the combustion zone to the mill; conveying a portion ofthe cooled gases to the mill; regulating the flow of these cooled gasesin response to said measure of heat transferred to control the quantityof heat transferred to steam; introducing air into the mill; introducingfuel into the mill; and conveying the products from the mill to thecombustion zone, for the burning of fuel therein.

7. The method of operation as in claim 6 including also the step ofregulating the air introduced to the mill to control mill airtemperature.

8. The method of operation as in claim 7 including also the step ofregulating the remaining gaseous mediums entering the mill to maintainthe volume flow of products leaving the mill essentially constant.

9. The method of operation as in claim 7 where the volume flow leavingthe mill is maintained essentially constant by allowing the portion ofhot gases being conveyed from the furnace to the mill, to assume aquantity which meets the mills natural volume demand.

10. In a steam generator having steam heating surfaces disposed therein,and a mill for the pulverizing and drying of high moisture content fuel,the method of operation comprising: burning fuel in a combustion zoneforming hot gases; passing these gases over steam heating surface totransfer heat to the steam and cool the gases; measuring the temperatureof the steam so heated; conveying a portion of the hot gases from thecombustion zone to the mill; conveying a portion of the cooled gases tothe mill; supplying air to the mill; feeding fuel into the mill;conveying the mill products to the combustion zone; controlling one ofthe gaseous mediums entering the mill in response to the steamtemperature to control steam temperature; controlling a second gaseousmedium entering the mill to control mill temperature; and controllingthe remaining gaseous medium entering the mill to control the volumeflow leaving the mill.

11. A vapor generator including a fluid cooled furnace, vapor heatingmeans disposed so that the gases produced in said furnace passthereover, means for firing a furnace with a high moisture content fuelincluding a gas swept pulverizing means the effluent of which isconveyed to and introduced into the furnace; means for supplying saidpulverizing means with a high moisture content fuel and with carryingand drying gases, with these latter including 7 8 air, hot gases fromthe furnace at a location upstream of References Cited by the Examinersaid vapor heating means, and gases which have traversed UNITED STATESPATENTS at least a portion of said vapor heating means; means forregulating the supply of gases which have traversed said vapor heatingmeans in response to the temperature of 2,812,747 11/1957 Armacost122479 FOREIGN PATENTS the heated vapor independently of the firingrate; means 1,202,239 7/1959 France. for regulating the supply of one ofthe remaining gases 312,707 6/1929 Great Britain. to control the flowrate of gases through said pulverizing 756,803 9/1956 Great Britain.

means; and means for controlling the other of said remaining gases tocontrol the temperature of the gases 10 JAMES WESTHAVE-R PrimaryExammer' leaving said pulverizing means. CHARLES J. MYHRE, Examiner.

1. A VAPOR GENERATOR COMPRISING IN COMBINATION A FURNACE HAVING HEATEXCHANGE TUBES ON ITS WALL, A GAS PASS EXTENDING FROM THE FURNACE, MEANSFOR HEATING VAPOR HAVING AT LEAST A PORTION THEREOF DISPOSED IN SAID GASPASS, MEANS FOR FIRING SAID VAPOR GENERATOR WITH A PARTICULATE FUEL OFHIGH MOISTURE CONTENT AND FOR REGULATING THE TEMPERATURE OF THE VAPORHEATED IN SAID VAPOR HEATING MEANS COMPRISING; A PULVERIZING MEANS, THEEFFLUENT FROM WHICH IS CONVEYED TO THE FURNACE, MEANS SUPPLYING SAIDFUEL TO THE PULVERIZING MEANS; MEANS PASSING A HOT GASEOUS MEDIUMTHROUGH THE PULVERIZING MEANS FOR FUEL ENTRAINMENT AND DRYING INCLUDINGMEANS CONVEYING THERETO HOT GASES FROM THE FURNACE, SEPARATE MEANSCONVEYING THERETO COMBUSTION GASES WHICH HAVE TRAVERSED SAID VAPORHEATING MEANS, MEANS CONVEYING THERETO AIR; AND MEANS REGULATING THESUPPLY OF GASEOUS MEDIUM TO THE PULVERIZING MEANS IN RESPONSE TO THETEMPERATURE OF THE VAPOR LEAVING SAID VAPOR HEATING MEANS AND TO MEETTHE TEMPERATURE AND FLOW REQUIREMENTS OF THE MILL.